Non-volatile memories usage is widely spreading as they allow reduction of the power consumption of the memory chips, due to the fact that they retain data without any external source. Flash memory is currently being used in a wide variety of devices but suffers from a limited endurance and lowers writing speed.
New types of memories are being developed to overcome these limitations. For example, Phase Change Random Access Memories (PCRAM) and Magnetic Random Access Memories (MRAM) have been identified by the International Technology Roadmap for Semiconductors (ITRS) as most adequate for flash memory replacement.
Typical MRAM structure is based on magnetic tunnel junctions using ferromagnetic materials separated by a thin insulator barrier through which electrons flow by tunnel effect. One of the ferromagnetic materials has its magnetization pinned (or also may be referred to as a pinned layer or a fixed magnetic layer) while the second ferromagnetic layer is set so that its magnetization can be switched from a direction parallel to the direction of the magnetization of the pinned layer (labeled as the P state) to a direction anti-parallel to the direction of the magnetization (i.e., magnetization orientation) of the pinned layer (labeled as the AP state). The second ferromagnetic layer may be referred to as the free layer or the free magnetic layer. The resistance of the AP state is higher than the resistance of the P state, allowing the system to store data as “1” for the high resistance state and “0” for the low resistance state.
Conventional MRAM require a magnetic field to be generated in order to write data (i.e., to switch the magnetization of the free layer) and suffer from a lack of scalability due to the current required to generate a high enough magnetic field at small dimensions. Passing polarized currents through a magnetic layer can reverse its magnetization, a phenomenon known as spin transfer torque. The effect of spin transfer torque forms the basis of spin torque transfer MRAM (STT-MRAM) and allows high scalability of the storage devices as the current required to write data decreases with the size of the MRAM cell. Typical STT-MRAM requires a current density of about 106 A/cm4 to write data and further reduction of this current density allows the development of low-power consumption devices, integrating with current CMOS technology node. However, writing current densities for pure spin-torque effect is approaching a limit.
Electric field assisted modification of the anisotropy of magnetic layers may help reach lower writing current densities. Hence, it is a need to have write and sense circuits configured for MRAM that utilizes a combination of spin torque transfer and electric field assisted anisotropy tuning.